1. Field of the Invention
The present invention relates to a motor driving device for driving a motor and a vacuum pump having the motor driving device.
2. Description of the Related Art
In vacuum pumps such as turbo-molecular pumps for rotating rotors at a high speed so as to carry out evacuation, DC brushless motors are frequently used as motors for driving to rotate rotors. In vacuum pumps that do not use rotation sensors, rotational speed information and magnetic pole position information of motor rotors necessary for rotation driving are estimated based on detection signals relating to a three-phase voltage and a three-phase current of motors.
In general, voltages that are modulated into a sinusoidal wave shape and modulated into a PWM signal by inverters are applied to motors. Such a voltage is generated by on/off control of a switching element, and has a voltage waveform of a rectangular wave that steeply changes. For this reason, a detected three-phase voltage detection signal and a three-phase current detection signal are allowed to pass through a low-pass filter so that a high frequency noise is removed. The rotational speed information and the magnetic pole position information of motor rotors are estimated by using the detection signals from which high frequency noises are removed.
However, when the low-pass filter removes the high frequency noises, a signal delay occurs in passed basic wave component (sinusoidal wave component). For this reason, an unignorable error is caused between an actual magnetic pole position and an estimated magnetic pole position, and stability is deteriorated in a control system where a cycle of rotation driving→voltage and current detection→magnetic pole position estimating operation is repeated, so that a higher harmonic wave that is the integral multiple of a rotational frequency increases. When the higher harmonic wave is overlapped with a basic wave of the rotational frequency, the basic wave is distorted from the sinusoidal wave. As a result, when an operating load is heavy, flow ripple of the motor current or step-out of the motor occasionally occurs.